Reminds users that the most popular OS on the planet has about 1,000 days to live

Microsoft on Monday made its most aggressive move yet to convince customers to drop Windows XP and adopt Windows 7, telling them that there were only 1,000 days of support life left in the older operating system.

Stephen Rose, the IT community manager for the Windows commercial team, noted the 1,000 days remaining for Windows XP support in a post to a Microsoft blog .

"Windows XP had an amazing run and millions of PC users are grateful for it. But it's time to move on," Rose said, adding that the operating system exits security support in "less than 1,000 days."

The 10-year-old XP actually has a little longer to live than that: Microsoft has promised to patch XP through April 8, 2014, 1,002 days from Monday.

"Bottom line, PCs running Windows XP will be vulnerable to security threats" after that date, said Rose. "Furthermore, many third-party software providers are not planning to extend support for their applications running on Windows XP, which translates to even more complexity, security risks, and ultimately, added management costs for your IT department."

According to usage statistics and research firm surveys, Microsoft has its work cut out in moving users off XP.

Web metrics firm Net Applications now has Windows 7 's usage share at 27%, for example, but XP still powers 51% of the world's personal computers. If the trends of each over the last three months continue, Windows 7 won't pass XP in the race for share until the second quarter of 2012.

Businesses are even more reliant on Windows XP, said Forrester Research when it recently estimated the aged OS's share at 60% of enterprise PCs .

Monday wasn't the first time Microsoft portrayed XP as yesterday's OS. Earlier this year, executives on the Internet Explorer (IE) team called XP the "lowest common denominator" as they explained why the OS wouldn't run IE9 or any future browsers .

And the company has taken firm steps to kill off other products it considers obsolete. Since mid-2009, Microsoft has urged users to give up IE6, the browser that shipped shortly before XP. Four months ago it upped the ante by launching a deathwatch website that highlights IE6's dwindling usage share.

The push to abandon XP coincided with the opening of Microsoft's Worldwide Partner Conference (WPC), the company's annual reseller meet. CEO Steve Ballmer kicked off WPC by celebrating another Windows 7 milestone: selling 400 million licenses for the OS.

Tami Reller, head of product marketing for the Windows group, cited that number to compare Windows 7's uptake with XP's in the same span of time.

"That is three times the pace of Windows XP," Reller said.

Unmentioned Monday -- for some time, actually -- was Windows Vista, the hapless 2007 version that has been called Microsoft's first OS failure since 2000's Windows Millennium. Customers agree: Vista peaked at just under 19% in October 2009, but has lost about half its share since.

Instead, Reller talked up not just Windows 7 as the replacement for XP, but also its successor, Windows 8, which most expect to ship next year.

While Reller encouraged corporate customers to continue deploying Windows 7, she promised that Windows 8 would run on the same hardware.

"For our business customers, your customers," she said, speaking to the partners at WPC, "this is an important element because the ability of Windows 8 to run on Windows 7 devices ensures that the hardware investments that these customers are making today will be able to take advantage of Windows 8 in the future."

While neither Reller nor Ballmer mentioned Windows 7's lifecycle, the company will push consumers now running Windows 7 to upgrade to Windows 8, too. According to Microsoft's longstanding practice, it will support Windows 7 Home Premium, the most popular edition for consumers, for five years, half the time slated for enterprise support.

Windows 7 Home Premium will be retired from security support in January 2015.